The term "sandwich panel" is used herein to designate a part constituted by two thin covering faces or "skins" that are interconnected by a core disposed between the two faces. The core is made up of stiffening partitions that extend perpendicularly to the skins and that define cells between one another. Various types of cellular cores are well known, e.g. honeycomb cores, corrugated cores, tubular cores, cup cores, . . .
In the field of cold composites, methods of bonding by means of adhesive between the skins and the core are commonly performed.
The same is not true of thermostructural composite material, i.e. materials such as carbon-carbon composites or ceramic matrix composites that have mechanical properties making them suitable for constituting structural elements and that are capable of conserving those properties at high temperatures.
There is a need for sandwich panels made of thermostructural composite materials, in particular in aviation and space applications, e.g. to constitute structural parts of space planes, hypersonic planes, or combined-propulsion planes.
Other applications can be envisaged, in particular for the blades and vanes of turbines, mirror supports having great dimensional stability, fairings suitable for being exposed to large heat flows, or fire-break partitions in aviation, marine, or land applications.
Various methods are indeed known that enable parts of complex shape to be fabricated from thermostructural composite material, by making separate preforms for different portions of such parts, assembling the preforms in a non-densified or an incompletely densified state, and then co-densifying the assembled-together preforms.
A known method for the making of sandwich panels, described in particular in Document EP-A-0 051 535, consists in depositing a thermolysable bonding agent by coating it between the facing faces of the preforms for the skins and for the core, before they have been fully densified, and then in thermoliyzing the bonding agent and co-densifying the skins, the core, and the bonding agent. That method suffers from the drawback of not enabling bonding quality to be controlled uniformly, where said quality is a function of the bonding agent used and of the specific surface area of bonding. In addition, the mass of the panel is increased by the presence of the bonding agent.
Another known method consists in implementing a textile type union by stitching or by implanting fibers, however a high density of stitches is required in order to avoid concentrating stresses at any particular stitch and in order to provide sufficient bonding.
It is also possible to consider a mechanical assembling of the differents components of a composite material part, for example by means of screws, possibly after the component preforms have been densified. Unions made in that way are effective, but they apply at points only. In the case of sandwhich panels, reducing stress concentrations means that complex interface shapes are required between the screws and the skins, together with the presence of inserts.
In another known process, described in document GB-A-1 387 868, the bonding of two components of a composite material part (fiber reinforced polyester) can be achieved by inserting a reinforcing element in the form of a film bearing a plurality of rigid needles extending perpendicularly to its surface. This type of uniting requires then a supplementary element which has to be separately manufactured.
Finally, in the making of a preform by superposition of plane fabric layers, a process is described in document FR-A-2 189 207 which consists in treating the surface of the fabric by abrasion to allow fibers to loosen. The number of contact points between fabric layers is thereby increased, which constitute growing points for the material constituting the matrix upon subsequent densification of the preform. An increased resistance to delamination in thus achieved This process is applied to elements of a preform, not to the uniting of already realized preforms. In addition a specific operation step is required, namely surface abrasion of the fabric layers, which has a destructive effect.